Conventionally, one type of telephone system has a cordless telephone set with a cordless receiver and enables a talking operation through a telephone line with this cordless telephone set.
Generally the telephone system as described above does not have a mechanical hook switch for having the cordless telephone set hooked.
As described above, the telephone system does not comprise the hook switch, so that the telephone system can not maintain an energized state with the subscriber's line. For this reason, as a countermeasure against power failure due to disconnection of the power supply from the commercial utility, power required for operations of a microcomputer is backed up by using a backup power such as a battery or an electric double-layered capacitor or the like.
In the telephone system according to the example based on the conventional technology described above, power is backed up by using a backup power source when power failure occurs, however, a battery or a capacitor supplies power thereto by means of discharge, so that a period of usable time of the battery or the like for backup is restricted depending on output capacity stored in the battery or the like.